The role of nitric oxide in the maintenance of vasoactive balance

Endothelial dysfunction may be considered as the interstage between risk factors and cardiovascular pathology. An imbalance between the production of vasorelaxing and vasoconstricting factors plays a decisive role in the development of hypertension, atherosclerosis and target organ damage. Except vasorelaxing and antiproliferative properties per se, nitric oxide participates in antagonizing vasoconstrictive and growth promoting effects of angiotensin II, endothelins and reactive oxygen species. Angiotensin II is a potent activator of NAD(P)H oxidase contributing to the production of reactive oxygen species. Numerous signaling pathways activated in response to angiotensin II and endothelin-1 are mediated through the increased level of oxidative stress, which seems to be in casual relation to a number of cardiovascular disturbances including hypertension. With respect to the oxidative stress, the NO molecule seems to be of ambivalent nature. On the one hand, NO is able to reduce generation of reactive oxygen species by inhibiting association of NAD(P)H oxidase subunits. On the other hand, when excessively produced, NO reacts with superoxides resulting in the formation of peroxynitrite, which is a free radical deteriorating endothelial function. The balance between vasorelaxing and vasoconstricting substances appears to be the principal issue for the physiological functioning of the vascular bed.     

Osteoporosis in experimental postmenopausal polyarthritis: the relative contributions of estrogen deficiency and inflammation

Generalized osteoporosis in postmenopausal rheumatoid arthritis (RA) is caused both by estrogen deficiency and by the inflammatory disease. The relative importance of each of these factors is unknown. The aim of this study was to establish a murine model of osteoporosis in postmenopausal RA, and to evaluate the relative importance and mechanisms of menopause and arthritis-related osteoporosis. To mimic postmenopausal RA, DBA/1 mice were ovariectomized, followed by the induction of type II collagen-induced arthritis. After the mice had been killed, paws were collected for histology, one femur for bone mineral density (BMD) and sera for analyses of markers of bone resorption (RatLaps; type I collagen cross-links, bone formation (osteocalcin) and cartilage destruction (cartilage oligomeric matrix protein), and for the evaluation of antigen-specific and innate immune responsiveness. Ovariectomized mice displayed more severe arthritis than sham-operated controls. At termination of the experiment, arthritic control mice and non-arthritic ovariectomized mice displayed trabecular bone losses of 26% and 22%, respectively. Ovariectomized mice with arthritis had as much as 58% decrease in trabecular BMD. Interestingly, cortical BMD was decreased by arthritis but was not affected by hormonal status. In addition, markers of bone resorption and cartilage destruction were increased in arthritic mice, whereas markers of bone formation were increased in ovariectomized mice. This study demonstrates that the loss of endogenous estrogen and inflammation contribute additively and equally to osteoporosis in experimental postmenopausal polyarthritis. Markers of bone remodeling and bone marrow lymphocyte phenotypes indicate different mechanisms for the development of osteoporosis caused by ovariectomy and arthritis in this model.     

Effect of estrogen on flow-induced dilation in NO deficiency: role of prostaglandins and EDHF.

To investigate the role of estrogen in flow-induced dilation (FiD) in nitric oxide (NO) deficiency, FiD was examined in isolated gracilis arterioles of ovariectomized (OVX) and OVX rats with estrogen replacement (OVE). Both groups of rats were treated chronically with N(omega)-nitro-L-arginine methyl ester. Plasma concentration of NO(2)/NO(3) was reduced in both groups. Plasma concentration of estradiol was lower in OVX than in OVE rats. FiD was similar in vessels of the two groups; calculated wall shear stress and basal tone were significantly greater in OVX vs. OVE rats. Indomethacin did not affect FiD in vessels from OVE rats but abolished dilation in vessels from OVX rats. Valeryl salicylate or NS-398 inhibited FiD by approximately 50%, whereas their simultaneous administration eliminated the response in arterioles from OVX rats. In vessels from OVE rats, miconazole or charybdotoxin eliminated FiD. Thus, in NO deficiency, prostaglandins derived from both cyclooxygenase isoforms mediate FiD in gracilis arterioles of OVX rats. Estrogen replacement switches the mediation, showing dependence on endothelium-derived hyperpolarizing factor in the arterioles of OVE rats.     

Changes in the homeostatic balance of prostacyclin-thromboxane generating systems during experimental social stress

Chronic emotional stress leads to the reduction in prostacycline content in the vascular wall and to the elevation of the malonic aldehyde level in the platelets, with a simultaneous increase of the spontaneous platelet aggregation index. Antistressor and antiaggregation action of the adaptation to high mountains occurs at later times (day 30) in both normal and stress-exposed rats despite the fact that stress-exposed rats yield to normal ones as regards the magnitude of prostacycline activation in the vascular wall. This is accounted for by initial (before adaptation) stress-induced disturbance of the balance between prostacycline and thromboxan.     

Altered regulation of renal nitric oxide and atrial natriuretic peptide systems in angiotensin II-induced hypertension

The present study was aimed to determine whether there is an altered role of local nitric oxide (NO) and atrial natriuretic peptide (ANP) systems in the kidney in association with the angiotensin (Ang) II-induced hypertension. Male Sprague-Dawley rats were used. Ang II (100 ng·min?1·kg?1) was infused through entire time course. Thirteenth day after beginning the regimen, kidneys were taken. The protein expression of NO synthase (NOS) and nitrotyrosine was determined by semiquantitative immunoblotting. The mRNA expression of components of ANP system was determined by real-time polymerase chain reaction. The activities of soluble and particulate guanylyl cyclases were determined by the amount of cGMP generated in responses to sodium nitroprusside and ANP, respectively. There developed hypertension and decreased creatinine clearance in the experimental group. The protein expression of eNOS, nNOS and nitrotyrosine was increased in the cortex, while that of iNOS remained unaltered. The urinary excretion of NO increased in Ang II-induced hypertensive rats. The catalytic activity of soluble guanylyl cyclase was blunted in the glomerulus in Ang II-induced hypertensive rats. The mRNA expression of ANP was increased in Ang II-induced hypertensive rats. Neither the expression of NPR-A nor that of NPR-C was changed. The protein expression of neutral endopeptidase was decreased and the activity of particulate guanylyl cyclase was blunted in the glomerulus and papilla in Ang II-induced hypertensive rats. In conclusion, the synthesis of NO and ANP was increased in the kidney of Ang II-induced hypertension, while stimulated cGMP response was blunted. These results suggest desensitization of guanylyl cyclase in the kidney of Ang II-induced hypertensive rats, which may contribute to the associated renal vasoconstriction and hypertension.     

Studies of the renal vasoactive systems in hyporeninemic hypoaldosteronism

Plasma renin activity, total renin, active renin, and aldosterone were measured as well as urinary prostaglandin E2 and kallikrein in a group of patients with hyperkalemia (6.1–7.7 mEq per liter) and hypo reninemic hypoaldosteronism. Plasma renin activity and aldosterone were low and the response was markedly blunted to upright posture, and furosemide. The rise in cortisol but not aldosterone was normal following ACTH stimulation. Active renin was depressed; however, total renin was normal. Urine PGE was variable including some low values, but the mean of the group was normal (p >0.1). Urine kallikrein excretion was markedly diminished and undetectable in most cases. Fludrocortisone normalized potassium but minimally increased kallikrein in the patients. The possibility exists that kallikrein deficiency in these patients may underlie the inability to generate active renin.     

The role of the reactions of NO with superoxide and oxygen in biological systems: A kinetic approach

In this study we calculate the half-life of ^·NO in its reactions with superoxide and with oxygen under various conditions using the known rate constants for these reactions. The measured half-life of ^·NO in biological systems is 3–5 s, which agrees well with the calculated value for intracellular ^·NO, but not for extracellular ^·NO under normal physiological conditions. The autoxidation of ^·NO to yield NO₂ as a final product cannot be responsible for such a short measured half-life under normal as well as pathologic conditions. Therefore, if there is direct evidence for the occurrence of the reaction of ^·NO with O₂ in the medium, one has to assume that the steady state concentrations of free ^·NO are much lower than those measured. The very low concentrations of free ^·NO in biological systems may result from its reversible strong binding to biological molecules. Simulation of the mechanism of the autoxidation of ^·NO indicates that the binding constants of ^·NO to O₂ or to another ^·NO are too small to account for the very low concentration of free ^·NO in biological systems. Nevertheless, the reaction of ^·NO with oxygen cannot be neglected in biological systems if the intermediate ONOO· reacts rapidly with a biological target. The biological damage caused by ONOO′ is expected to be due to the radical itself and to peroxynitrite, which is most probably formed via the reaction of ONOO· with the biological molecule.     

Pulmonary hypertension: the role of the electrocardiogram

A 54-year-old female was referred to our centre for further evaluation of recently established severe pulmonary hypertension. Six months prior to presentation to the cardiologist of the referring centre, the patient had first experienced exertional dyspnoea. At the time of presentation to the referring cardiologist, the patient’s ECG showed signs of an increased right heart load. Interestingly, this patient had undergone a thorough cardiac evaluation in the referring centre seven years before when she presented with severe hyperthyroidism. At that time there were no symptoms or signs of pulmonary hypertension on ECG, echocardiography, or at heart catheterisation. Thorough evaluation in cooperation with the referring centre demonstrated that this patient was suffering from idiopathic pulmonary arterial hypertension, a rare form of pulmonary hypertension. We conclude this report with a discussion on the potential use of the ECG for the diagnosis of increased right heart load. (Neth Heart J 2008;16:250-4.)     

Morphological and functional changes in experimental ocular hypertension and role of neuroprotective drugs

Glaucoma is a neurodegenerative disease characterized by progressive loss of retinal ganglion cell axons and their cell bodies in the retina. Elevated intraocular pressure (IOP) is considered to be the major risk factor associated with the development of this neuropathy. Randomized controlled clinical trials have demonstrated that in some patients the disease progresses, even after lowering the IOP. Several researchers have devised ways to induce elevated IOP in the rat eye with the aim of impeding the flow of aqueous humour out of the eye. Chronic ocular hypertension in rats induces morphofunctional changes in the optic nerve head and retina. Death of ganglion cells is thought to follow an apoptotic pathway. Changes have also been reported in neuronal and non-neuronal cells, levels of cyclooxygenase, and nitric oxide synthase, endothelin 1 and brain derived neurotrophic factor. Other mechanisms include intracellular electrolyte imbalance, microglial phagocytosis and elevated glutamate levels. Neuroprotection is the treatment strategy by preventing neuronal death. Hypotensive drugs (beta-blockers, alpha-agonists and prostaglandins), Ca++ channel blockers, NMDA antagonists and nitric oxide synthase inhibitors have been used as neuroprotective drugs in experimental models of glaucoma.     

Distal-to-proximal NO conversion in hemoproteins: the role of the proximal pocket

Hemoproteins play central roles in the formation and utilization of nitric oxide (NO) in cellular signaling, as well as in protection against nitrosative stress. Key to heme-nitrosyl function and reactivity is the Fe coordination number (5 or 6). For (five-coordinate) 5c-NO complexes, the potential for NO to bind on either heme face exists, as in the microbial cytochrome c′ from Alcaligenes xylosoxidans (AxCYTcp), which forms a stable proximal 5c-NO complex via a distal six-coordinate NO intermediate and a putative dinitrosyl species. Strong parallels between the NO-binding kinetics of AxCYTcp, the eukaryotic NO sensor soluble guanylate cyclase, and the ferrocytochrome c/cardiolipin complex have led to the suggestion that a distal-to-proximal NO switch could contribute to the selective ligand responses in gas-sensing hemoproteins. The proximal NO-binding site in AxCYTcp is close to a conserved basic (Arg124) residue that is postulated to modulate NO reactivity. We have replaced Arg124 by five different amino acids and have determined high-resolution (1.07-1.40 Å) crystallographic structures with and without NO. These, together with kinetic and resonance Raman data, provide new insights into the mechanism of distal-to-proximal heme-NO conversion, including the determinants of Fe-His bond scission. The Arg124Ala variant allowed us to determine the structure of an analog of the previously unobserved key 5c-NO distal intermediate species. The very high resolution structures combined with the extensive spectroscopic and kinetic data have allowed us to provide a fresh insight into heme reactivity towards NO, a reaction that is of wide importance in biology.     

A DELLAcate balance: the role of gibberellin in plant morphogenesis

The importance of gibberellin (GA) in vegetative and reproductive development has been known for some time. Recent studies have uncovered new roles of GA in leaf differentiation, photomorphogenesis and pollen-tube growth. Significant contributions to our understanding of GA-regulated morphogenesis include the identification of upstream regulators of GA biosynthesis, the elucidation of the function of GA signaling components, and the isolation of downstream targets. In addition, the mechanisms of interactions between GA and other hormone pathways are beginning to be revealed at the molecular level.     

Water balance in Hemizonia luzulifolia: the role of extracellular polysaccharides

Abstract. The role of extracellular water in ameliorating drought stress was examined in Hemizonia luzulifola , an annual composite that comprises two subspecies differing significantly in the amount of extracellular polysaccharide within basal leaves. Rosette leaves of the high polysaccharide (HP) ssp were more than 30% pectin on a dry weight basis in contrast to only 4% in the low polysaccharide (LP) ssp. Concomitant with this difference in polysaccharide content was a significant difference in the dehydration response of leaves. Near full hydration, relative capacitances of HP leaves (0.7 MPa^-1) were an order of magnitude greater than the LP leaves (0.08 MPa^-1). Relative capacitance of the polysaccharide alone was 1.5 MPa^-1. The weight of water per unit leaf area was not significantly different in the two taxa. Nevertheless, the pectin-like polysaccharides significantly altered the distribution of metabolically available water from primarily cell-stored water in the LP ssp to apoplasmic and symplasmic capacitors in leaves of the HP ssp. The consequence of this apoplasmic water store with colloidal properties is that the Ψ-dependence of leaf capacitance ceases to be linked directly to cell water relations. Transfer resistances for water movement between capacitors and the xylem near full hydration (0 to -0.5 MPa) were significantly larger in the HP leaf. This difference in transfer resistance was interpreted to be the result of a large resistance to water movement between the polysaccharide and the xylem. Because of these large transfer resistances, the apoplasmic capacitor probably buffers cells at lower water potentials under transient water fluxes than expected from laboratory measurements made during slow desiccation. Field measurements support this conclusions; HP leaves were better buffered than LP leaves at midday water potentials.     

The NO world for plants: achieving balance in an open system

Nitric oxide (NO) is a free radical that had been known for many years simply as a toxic air pollutant. The discovery of enzymatic NO production in many living organisms has established a new paradigm: NO being an essential molecule endogenously produced in the cells. In plant science it has been suggested that NO acts as a plant hormone equivalent to ethylene; that is, as a gaseous signal transmitter. Even after experiencing such a scientific breakthrough, however, researchers may still feel difficulty in exploring plant NO signalling systems with conventional approaches. A major difference between plants and animals is that the growth and development of plants is closely linked to the surrounding environment where NO levels vary according to biotic and abiotic activities. This fundamental difference may make the NO-signalling network system of plants larger and more complicated than that of vertebrates. This review intends to show prospects for the future of NO signalling research in plants by introducing a holistic concept to aid in the exploration of complicated systems such as the plant-environment system. Furthermore, the novel ONS hypothesis is proposed to encompass the complexity and simplicity of NO in chemistry, biochemistry and physiology.     

Endothelial alterations during inhaled NO in lambs with pulmonary hypertension: implications for rebound hypertension

Clinically significant increases in pulmonary vascular resistance (PVR) have been noted upon acute withdrawal of inhaled nitric oxide (iNO). Previous studies in the normal pulmonary circulation demonstrate that iNO increases endothelin-1 (ET-1) levels and decreases endogenous nitric oxide synthase (NOS) activity, implicating an endothelial etiology for the increase in resistance upon iNO withdrawal. However, the effect of iNO on endogenous endothelial function in the clinically relevant pulmonary hypertensive circulation is unknown. The objective of this study was to determine the effects of iNO on endogenous NO-cGMP and ET-1 signaling in lambs with preexisting pulmonary hypertension secondary to increased pulmonary blood flow. Eight fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt lambs). After delivery (4 wk), the shunt lambs were mechanically ventilated with iNO (40 ppm) for 24 h. After 24 h of inhaled NO, plasma ET-1 levels increased by 34.8% independently of changes in protein levels (P < 0.05). Contrary to findings in normal lambs, total NOS activity did not decrease during iNO. In fact, Western blot analysis demonstrated that tissue endothelial NOS protein levels decreased by 43% such that NOS activity relative to protein levels actually increased during iNO (P < 0.05). In addition, the beta-subunit of soluble guanylate cyclase decreased by 70%, whereas phosphodiesterase 5 levels were unchanged (P < 0.05). Withdrawal of iNO was associated with an acute increase in PVR, which exceeded baseline PVR by 45%, and a decrease in cGMP concentrations to levels that were below baseline. These data suggest that the endothelial response to iNO and the potential mechanisms of rebound pulmonary hypertension are dependent upon the underlying pulmonary vasculature.